Genetics: DNA Replication, Chromosome Structure, Linkage, Mutations, Sex Determination, and Gene Expression

Comprehensive review flashcards covering DNA replication, chromosome structure, linkage and mapping, chromosomal aberrations, sex determination mechanisms, and gene expression.

Based on the Meselson-Stahl experiment shown below, what experimental technique was used to distinguish between DNA containing $^{15}\text{N}$ and $^{14}\text{N}$, and which model of replication did the results support?

The experiment used the sedimentation equilibrium centrifugation technique; the results demonstrated that bacterial DNA replication is semiconservative.

What is the single origin of replication in Escherichia coli, and how large is the resulting replicon?

The origin of replication is $oriC$, and the entire circular chromosome constitutes a single replicon of $4.6 \times 10^6$ base pairs ($4.6\text{ million base pairs}$).

How do the exonuclease activities of bacterial DNA Polymerase I and DNA Polymerase III differ?

Both DNA Polymerase I and III possess $3'\text{--}5'$ exonuclease activity for proofreading; however, only DNA Polymerase I has $5'\text{--}3'$ exonuclease activity, which is used to remove RNA primers and fill gaps.

Identify the primary subunits and assemblies that form the bacterial DNA Polymerase III holoenzyme shown below.

The holoenzyme consists of the DNA Polymerase III core enzyme complexes (catalytic function), the $\tau$ (tau) subunits, the sliding DNA clamp loader, and the sliding DNA clamp.

What are the specific functions of DnaA, DNA helicase, single-stranded binding (SSB) proteins, and DNA gyrase during bacterial DNA replication?

DnaA binds to the origin ($oriC$) to open the helix; DNA helicase denatures hydrogen bonds using ATP hydrolysis; SSB proteins bind and protect single DNA strands; and DNA gyrase (a topoisomerase) makes cuts to relieve coiling tension and reseals the strand.

Which three eukaryotic DNA polymerases are directly involved in nuclear DNA replication, and what are their respective roles?

DNA polymerase $\alpha$ (alpha) synthesizes primers; DNA polymerase $\varepsilon$ (epsilon) synthesizes the leading strand continuously; and DNA polymerase $\delta$ (delta) synthesizes Okazaki fragments on the lagging strand.

What is the human telomeric repeat sequence, and how does telomerase prevent chromosome shortening?

The human telomere sequence is $5'\text{--TTAAGGG--}3'$. Telomerase utilizes an intrinsic RNA guide to add repeated sequences to the $3'$ overhang, facilitating completion of the opposite strand.

In Avery, MacLeod, and McCarty's 1944 transformation experiment shown below, treatment with which enzyme abolished the transforming activity of the active factor?

Deoxyribonuclease (DNase) abolished transformation, confirming that DNA is the transforming principle.

In the 1952 Hershey-Chase blender experiment shown below, which radioactive isotope entered bacterial cells and directed phage reproduction?

$^{32}\text{P}$, which labels nucleic acids (DNA), entered the bacteria and appeared in viable progeny phages, while $^{35}\text{S}$-labeled protein coats remained outside as phage ghosts.

What structural feature at the C-$2'$ position distinguishes ribose from 2-deoxyribose?

Ribose has a hydroxyl group ($-\text{OH}$) at the C-$2'$ carbon, whereas 2-deoxyribose has a hydrogen atom ($-\text{H}$).

What critical structural feature of DNA was demonstrated by the X-ray diffraction pattern obtained by Rosalind Franklin and Maurice Wilkins?

The scatter pattern showed characteristic periodicity, indicating that DNA has a double-helical structure.

What quantitative base composition relationships did Erwin Chargaff discover between 1949 and 1953?

He discovered that the amount of adenine equals thymine ($\text{A} = \text{T}$), the amount of guanine equals cytosine ($\text{G} = \text{C}$), the sum of purines equals pyrimidines ($\text{A} + \text{G} = \text{C} + \text{T}$), while the percentage of $(\text{G} + \text{C})$ does not necessarily equal that of $(\text{A} + \text{T})$.

Which basic amino acids are abundant in histones, and how do they allow chromatin packaging?

Lysine and arginine; their positive charges form electrostatic bonds with the negatively charged phosphate groups along the DNA backbone.

When two heterozygous genes linked in coupling and repulsion exhibit complete linkage, what phenotypic ratio is produced in the $F_2$ generation?

A unique $1:2:1$ phenotypic linkage ratio.

How is one map unit ($\text{mu}$) defined in chromosome mapping?

$1\text{ map unit (mu)} = 1\%$ recombination frequency between two linked genes on a chromosome.

What formulas define the coefficient of coincidence ($C$) and genetic interference ($I$)?

$C = \frac{\text{Observed DCO}}{\text{Expected DCO}}$ and $I = 1.0 - C$.

How is the middle gene determined in a three-point mapping cross?

Compare the noncrossover (NCO) phenotypes (highest frequency) to the double crossover (DCO) phenotypes (lowest frequency); the single gene whose marker is flipped/different between the parental and double crossover arrangements is located in the middle.

Define monosomy, trisomy, and polyploidy in terms of the haploid chromosome number $n$.

Monosomy is $2n - 1$; trisomy is $2n + 1$; polyploidy represents organisms with three or more full haploid sets ($3n, 4n, 5n, \dots$).

What is haploinsufficiency?

A condition where a single functional copy of a gene in a diploid organism is insufficient to provide the normal or life-sustaining function.

What chromosomal abnormality causes Patau syndrome, and what are its primary clinical features?

Trisomy 13 ($47, 13+$); features include intellectual disability, microcephaly, cleft lip and palate, polydactyly, low-set deformed ears, congenital heart defects, and double ureter/kidney cysts.

What genetic defect is responsible for Cri du Chat syndrome in humans?

A deletion of a variable portion of the short arm of chromosome 5, designated as $46, 5\text{p}-$.

What is the structural difference between a paracentric inversion and a pericentric inversion?

A paracentric inversion does not involve the centromere, whereas a pericentric inversion includes the centromere within the inverted chromosomal segment.

In an individual heterozygous for a reciprocal translocation, what gametes are produced by alternate segregation versus adjacent segregation?

Alternate segregation produces normal and balanced gametes; adjacent segregation produces genetically unbalanced gametes containing duplications and deficiencies.

What trinucleotide repeat sequence in the FMR1 gene causes Fragile-X syndrome, and how many repeats indicate an affected individual?

The repeat is $\text{CGG}$; normal individuals have $6\text{--}54$ repeats, carriers have $55\text{--}230$ repeats, and affected individuals exhibit over $230$ repeats.

Describe sex determination in the insect Protenor as illustrated below.

Females have $14$ chromosomes ($12\text{A} + 2\text{X}$) and produce uniform $6\text{A} + \text{X}$ gametes (homogametic); males have $13$ chromosomes ($12\text{A} + \text{X}$) and produce $6\text{A}$ and $6\text{A} + \text{X}$ gametes (heterogametic), yielding a $1:1$ sex ratio.

What are the chromosome karyotypes and sexual phenotypes of Klinefelter and Turner syndromes?

Klinefelter syndrome is $47,\text{XXY}$ (male differentiation with underdeveloped testes and tall stature); Turner syndrome is $45,\text{X}$ (female differentiation with rudimentary ovaries and short stature).

According to the Lyon hypothesis, how many Barr bodies are found in a diploid human somatic cell with $N$ X chromosomes?

$N - 1$ Barr bodies; all but one X chromosome are inactivated and condensed into heterochromatic sex chromatin bodies.

How is sex determined in Drosophila melanogaster compared to humans?

In Drosophila, sex is determined by the ratio of X chromosomes to autosome sets ($\text{X}:\text{A}$), where $\text{X}:\text{A} = 1.0$ is female and $\text{X}:\text{A} = 0.50$ is male; the Y chromosome does not determine maleness, whereas in humans the Y chromosome (specifically $SRY$) determines maleness.

What role does the enzyme aromatase play in temperature-dependent sex determination (TSD) in reptiles?

Aromatase converts androgens (such as testosterone) into estrogens (such as estradiol), with its activity and transcription modulated by incubation temperature.

What are the start codon and the three standard stop codons in the mRNA genetic code?

The start codon is $\text{AUG}$ (coding for methionine); the stop codons are $\text{UAA}$, $\text{UAG}$, and $\text{UGA}$ (which do not encode any amino acid).

What is the role of the $\sigma$ (sigma) subunit in bacterial RNA polymerase holoenzyme transcription?

The $\sigma$ factor recognizes and binds specific promoter sequences to initiate transcription, then dissociates from the core enzyme once elongation begins.

What are the two consensus promoter sequences recognized by bacterial RNA polymerase in Escherichia coli?

The $-35$ consensus sequence ($\text{TTGACA}$) and the $-10$ consensus sequence ($\text{TATAAT}$, or Pribnow box).

What distinct RNA molecules are synthesized by eukaryotic RNA Polymerases I, II, and III, and where are they located?

RNA Polymerase I synthesizes rRNA in the nucleolus; RNA Polymerase II synthesizes mRNA and snRNA in the nucleoplasm; RNA Polymerase III synthesizes $5\text{S rRNA}$ and tRNA in the nucleoplasm.

What invariant dinucleotides mark the $5'$ donor and $3'$ acceptor sites of eukaryotic introns during splicing?

Introns begin at the $5'$ end with a $\text{GU}$ donor sequence and terminate at the $3'$ end with an $\text{AG}$ acceptor sequence.